scripts: use CT_CONFIG_SHELL instead of CT_SHELL

CT_SHELL is undefined.
Thus, the generated wrapper scripts are not executable by the kernel
because they do not contain a valid interpreter.
Use CT_CONFIG_SHELL instead.

Signed-off-by: "Titus von Boxberg" <titus@v9g.de>
(transplanted from cf509170838f4e289fe25617e3f3db1d6d6ffc41)

 reporter_name="Nye Liu"

 reporter_url="http://sourceware.org/ml/crossgcc/2008-10/msg00046.html"

 reporter_comment=""